1. Field
The present disclosure relates generally to fiber optics, and in particular, to a method and apparatus for precision tuning an optical element.
2. The Prior Art
Background
Optical filters formed using thin-film dielectric coatings are critical to the optical devices of today. As optical thin-film coating technologies improve, a greater array of optical filters is becoming available.
FIG. 1 shows a diagram of a prior art optical filter. Typically, optical filters are formed on a large substrate, such as substrate 100 of FIG. 1. Substrate 100 may range in diameter from a few to several centimeters, and may contain several laminates formed together to meet the desired specification. Depending on the type of laminates used, substrate 100 may range in thickness from 1 to 4 mm. Within substrate 100 several smaller dice are formed which represent individual optical filters, such as die 102 of FIG. 1. The individual die may be square, rectangular, or circular, and are typically 1-2 mm in diameter.
Manufacturers typically prepare optical filters to match a particular specification desired by a customer. One such specification is the center wavelength (CWL) of the filter. As is appreciated by those of ordinary skill in the art, the CWL is crucial to the operation of modern optical devices, such as dense wavelength division multiplexers. Customers typically specify the CWL to be within +/−50 ppm to +/−100 ppm of the ITU grid.
However, such accuracy is very difficult for optical filter manufacturers to reach, and thus the yield of dice meeting a particular CWL is typically very low. In fact, a yield as low as 5-10% is typical for a given substrate. Such a low yield has kept the price of thin-film filters relatively high.
Typically, the CWL for an optical filter is specified at the center of the filter. For example, manufacturers of devices such as thin-film filters typically specify the center wavelength only at the physical center of the filter. This is done because generally optical filters are disposed within optical devices such that light is incident at the center of the optical filter. Thus, if the customer's desired specification is not met at the center of the filter, the filter will have to be rejected.
In an effort to increase the yield of optical filters, the prior art used the well known fact that the CWL of an optical filter is dependent in the angle of incident light. If an optical filter has a CWL slightly out of specification, the optical filter may be angled to adjust the angle of incidence and thus the CWL of the filter. If the CWL was not too far out of specification, tilting the filter might bring the CWL of the filter back into specification.
However, tilting optical filters in applications such as dense wavelength division multiplexing is disfavored since the light reflected from the filter is often utilized as well as the filtered light passing through the filter. If the filter is tilted in an attempt to adjust the CWL, often the reflected light cannot be aligned correctly.